In the manufacture of integrated circuits, deposition of films or materials with non-conformal properties over retrograde topography often yields subsequent structures which also have retrograde or re-entrant profiles. Subsequent depositions of non-conformal and other films over re-entrant profiles frequently results in structures having an "overhang" which obscures the underlying topography.
Retrograde profiles may result from the deposition of: non-conformal films over ideally vertical profiles, and conformal films over profiles which are already retrograde.
Some manufacturing applications involve depositing a material over features which may have retrograde or re-entrant profiles, and then patterning and etching the material. Anisotropic etches (i.e., etches exhibiting no significant undercut) are used to prevent significant critical dimension loss. "Critical dimension" referring to the distance between features.
Anisotropic etches remove material in a direction perpendicular to the plane defined by the substrate. The material which is etched, must therefore be in the "line of sight" of the plasma, as viewed from a point directly "above" the feature. Consequently, any overhanging topography will shelter the material to be anisotropically etched. If conductive materials, such as polysilicon form the layers, conductive shorts may result between adjacent features as a consequence of the residual sheltered material. The residual material is referred to as a "stringer," as seen in FIG. 1.
"Stringers" can be defined as residual material resulting from an etch process. The "stringers" are a problem when they are comprised of a conductive material which causes short circuiting between adjacent structures.
Ion sputter etching has been found to have a characteristic etch pattern. This etch pattern makes reactive ion sputter etching a useful tool in forming faceted edges. The graph in FIG. 2 illustrates the Etch Angle versus Yield of the etch. Note that the Yield is highest at an angle of approximately 45.degree.. FIG. 3 illustrates the facet which results at the edges of a feature during the etch process. The facet angle is also approximately 45.degree..
In sputter etch, ions which impinge on horizontal surfaces have a minimal effect on etch rate and profile. However, the sputter yield of the etch at the corners is approximately four times that of the etch rate of a horizontal surface, thereby creating an extreme etch profile. The effect is the wearing away of the corners of a feature at approximately 45.degree. angles. The material removed by the sputter etch is redeposited along the sides of the feature and along the surface of the substrate. The amount of material redeposited is also effected by the actual shape of the feature being etched. Additionally, the redeposited material acts to fill cavities present in retrograde structures.
Current stacked capacitor dynamic random access memories (DRAMS) often comprise components which have high, vertical (retrograde) topologies. Such topologies are susceptible to "stringers" following etch steps. Since DRAMs are comprised of polysilicon, which is a conductive material, any "stringers" may result in non-functional parts and consequent yield loss.